DAVID GREENE, HOST:
OK. This week we have been contemplating, you know, just the universe. We wanted to put things in perspective. And today, we are looking at what the universe is made of. You would think this is one of the easier questions about the universe to answer, but NPR's Nell Greenfieldboyce says no.
NELL GREENFIELDBOYCE, BYLINE: You and I are made of atoms. Our whole planet is made of atoms. Look around you and everywhere you look, you see more atoms. It's the stuff that makes up planets and stars and galaxies, but this kind of stuff is just a tiny, tiny fraction of the universe. Sean Carroll is a theoretical physicist at Caltech.
SEAN CARROLL: All of the stuff we've ever seen in the laboratory, all the kinds of particles and matter and energy, that only makes up 5 percent of our universe.
GREENFIELDBOYCE: Five percent. So what is the rest of the universe made of? Well, one biggie is something called dark matter. About 25 percent of the universe is dark matter. Chuck Bennett is an astrophysicist at Johns Hopkins University. He says this stuff is literally dark.
CHUCK BENNETT: It just doesn't interact with light at all. It doesn't give off any light. It doesn't absorb light. It doesn't scatter light. There's no way to see it. The only way we know that it's there is because it has gravitational effects.
GREENFIELDBOYCE: Scientists discovered dark matter when they looked at the motion of galaxies and realized that something unseen had to be exerting a gravitational pull. Dark matter may be some kind of particle that we just haven't detected yet. The rest of the universe, 70 percent, is something even more crazy called dark energy. It appears to be some kind of energy that's inherent to empty space. It acts to push the universe apart, speeding up its expansion. Bennett says, like dark matter, dark energy is another big mystery.
BENNETT: We call dark matter dark matter because we don't know what it is (laughter). And we call dark energy dark energy because we don't know what it is, either. So other than the fact that we don't quite understand 95 percent of the universe, we're doing really well.
GREENFIELDBOYCE: All of the world's leading theoreticians who write whole books about the universe just have to live with this. Jim Peebles is the Albert Einstein Professor of Science Emeritus at Princeton University.
JIM PEEBLES: You're entitled to say if you're so smart, why don't you tell me what the dark matter is? And I'll have to confess I don't know.
GREENFIELDBOYCE: Do you find it depressing that so much of the universe is unknown?
PEEBLES: No. I think I'd be depressed if everything were nearly all known.
GREENFIELDBOYCE: He sees no danger of that anytime soon. Nell Greenfieldboyce, NPR News.
GREENE: And tomorrow, we'll look at whether our universe is the only one.
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